Definition of "write in" []

• To write a letter to, e.g. a publication. (verb)
• To fill in something required, by writing. (verb)

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• "The Learning Project at Park Tudor School in Indianapolis made a very effective addition to its assignment sheet by adding a section wherein the student could write in his evening and weekend plans, appointments, and activities."
• "While there is no suggestion that there was a secret enclave of warrior queens within the Templar Order, as Michael Baigent and Richard Leigh write in The Temple and the Lodge 1989:"
• "I hope it will also be my English school this September, after I take the entrance exam, and with audacity, I write in my notebook for Irina Petrovna, “I go to school by Bus 22.”"